In apparatus or tooling for forming end panels or shells for metal cans or plastic containers, for example, as disclosed in U.S. Pat. Nos. 4,093,102, 4,587,825, 4,587,826 and 4,637,961, it is desirable to construct the tooling so that the shells are produced from sheet metal or aluminum having a minimum gage or thickness. On the other hand, it is necessary for each shell to have sufficient strength for withstanding a predetermined pressure within the can without deforming or buckling. It is also desirable for the tooling to provide for high volume production of the shells on either a single or multiple action press and to complete the forming of each shell at a single station in order to avoid complicated reforming operations. Commonly, an end panel or shell includes a circular center panel which is connected by a panel radius to a U-shaped countersink portion having a countersink radius. The countersink portion is connected by a tapering or frusto-conical chuck wall portion to an upper crown portion which extends outwardly to a depending peripheral lip portion.
One of the common problems encountered in producing end panels or shells is the stretching and thinning of the sheet metal when forming a small panel radius and a small countersink radius. If there is stretching and thinning of the sheet metal in these areas, the strength of the shell rapidly decreases, with the result that the shells are unacceptable for use. The stretching and thinning of the sheet metal around the panel radius and countersink radius usually result from tooling which draws the chuck wall and center panel from the sheet metal or draws the center panel after drawing the chuck wall with a reforming operation, such as disclosed in the above-mentioned patents.